Protococcacea? 



193 



have been recorded, but there is much doubt about the observations, and 

 under normal circumstances motile cells do not occur in Protococcus viridis. 



Other species of Protococcus are not so well known and are less abundant. 

 P. rufescens is of a brick-red colour owing to the presence of hsematochrome 

 which is dissolved in an oil. P. dissectus is characterized by the way in 

 which the mother-cells divide, and the consequent disposition of the cells 

 in the colonies. In the Antarctic there are several distinct and conspicuous 

 species which are for the most part attached to papery sheets of the Blue- 

 green Alga Phormidium (W. & G. S. W., '11; Fritsch, '12). These Antarctic 

 forms are considerably larger than the species of more temperate regions, the 

 cells of P. antarcticus forma robusta attaining a diameter of 100 p. They 

 possess a massive parietal chloroplast, with or without pyrenoids ; they often 

 store numerous small starch grains and not infrequently a fatty oil, the latter 

 occurring sometimes in such quantity as to obscure all cytological details. 



K 



Fig. 119. A F, Trochiscia aspera (Eeinsch) Hansg. ; A and B, vegetative cells; C and D, 

 division of protoplast to form gonidia ; K, empty cell from which gonidia have escaped ; 

 F, palmella state. G and //, T. hirta (Eeinsch) Hansg. / and J, T. pttncixpinosa W. West ; 

 K, T. reticularis (Keinsch) Hansg. All x 520. z<j, zoogonidia found in a culture of 

 T. tispera : there is no positive evidence that they belong to that species, or even to the 

 genus Trochiscia. 



A genus not far removed from Protococcus is Trochiscia (fig. 119). There 

 are many species, some occurring in water and others on damp ground, but 

 very little is known about most of them. They differ from the species of 

 Protococcus in the external ornamentation of the cell-wall, which may be 

 areolated or thickly clothed with denticulations or spines. Most of them 

 are purely aquatic in habit. There are one or more parietal chloroplasts 

 in each cell, and pyrenoids are frequently present. Reproduction is mostly 

 by the formation of non-motile gonidia (aplanospores), 8 or 16 of which are 

 formed in a mother-cell. There is no conclusive evidence that zoogonidia 

 occur in this genus, although a palmella-state occurs in T. aspera (fig. 119 F). 

 Wille ('01 A ; '09) has stated that species of this genus are very probably the 

 resting zygotes of members of the Chlamydomonadeie, but available evidence 

 does not support this view. His reference of the genus to the Volvocacese is 



13 



W. A. 



